Imagine how much better it would be if it was 9MP. That would be 50% larger photocells and that would lead to better ISO performance, even at ISO100.
If it doesnt convert any aditional photons to pixels it wouldnt be any better.
If your using 100% of the sensor for collecting photons (which the 7D is close to, with gapless microlenses close to the photosites etc) combining those sites doesnt make any better an image.
it just looses detail. Theres no proof anywhere that with canons tech more pixels hurts anything, there producingthe best crop sensors in the market and the highest number of pixels.